Semiconductor device with cell trench structures and contacts and method of manufacturing a semiconductor device

ABSTRACT

First and second cell trench structures extend from a first surface into a semiconductor substrate. The first cell trench structure includes a first buried electrode and a first insulator layer between the first buried electrode and a semiconductor mesa separating the first and second cell trench structures. A capping layer covers the first surface. The capping layer is patterned to form an opening having a minimum width larger than a thickness of the first insulator layer. The opening exposes a first vertical section of the first insulator layer at the first surface. An exposed portion of the first insulator layer is removed to form a recess between the semiconductor mesa and the first buried electrode. A contact structure is in the opening and the recess. The contact structure electrically connects both a buried zone in the semiconductor mesa and the first buried electrode and allows for narrower semiconductor mesa width.

BACKGROUND

Semiconductor devices based on vertical IGFET (insulated gate fieldeffect transistor) cells include cell trench structures with buriedelectrodes and semiconductor mesas between the cell trench structures.Typically a photolithographic mask defines placement and size of thecell trench structures and another photolithographic mask definesplacement and size of contact structures providing electric contacts toimpurity zones in the semiconductor mesas. Other approaches rely onforming the contact structures self-aligned to the cell trenchstructures. It is desirable to provide semiconductor devices with narrowsemiconductor mesas and small distances between neighboring cell trenchstructures in a reliable way and at low costs.

SUMMARY

According to an embodiment, a method of manufacturing a semiconductordevice is provided. First and second cell trench structures are providedthat extend from a first surface into a semiconductor substrate. Thefirst cell trench structure includes a first buried electrode and afirst insulator layer between the first buried electrode and asemiconductor mesa separating the first and second cell trenchstructures. A capping layer is provided that covers the first surface.The capping layer is patterned to form an opening having a minimum widththat is larger than a thickness of the first insulator layer. Theopening exposes a first vertical section of the first insulator layer atthe first surface. An exposed portion of the first insulator layer isremoved to form a recess between the semiconductor mesa and the firstburied electrode. A contact structure is provided in the opening and therecess.

According to another embodiment, a method of manufacturing asemiconductor device includes introducing first and second cell trenchesfrom a first surface into a semiconductor substrate, wherein firstsemiconductor mesas are formed between first and second cell trenchesand second semiconductor mesas are formed between the first celltrenches. A first insulator layer is provided that lines at leastsidewalls of the first cell trenches. First buried electrodes areprovided in the first cell trenches on the first insulator layer. Acapping layer is provided that covers the first surface. The cappinglayer is patterned to form first and second openings having a minimumwidth that is larger than a thickness of the first insulator layer. Thefirst openings expose first vertical sections of the first insulatorlayers adjoining the first semiconductor mesas, respectively. The secondopenings expose the first buried electrodes of first cell trencheslocated between second semiconductor mesas. Exposed portions of thefirst insulator layers are removed to form recesses between the firstsemiconductor mesas and adjoining ones of the first buried electrodes.Conductive material is deposited to form, in the recesses and the firstopenings, first contact structures and second contact structures in thesecond openings.

Another embodiment refers to a semiconductor device with first andsecond cell trench structures extending from a first surface into asemiconductor substrate. First semiconductor mesas separate first andsecond cell trench structures and second semiconductor mesas separatefirst cell trench structures. The first cell trench structures include afirst buried electrode and a first insulator layer respectively, whereinfirst vertical sections of the first insulator layers separate the firstburied electrodes from the first semiconductor mesas. A capping layer islocated on the first surface. The semiconductor device includes firstcontact structures, wherein each first contact structure includes afirst section in an opening of the capping layer and a second sectionbetween one of the first semiconductor mesas and one of the first buriedelectrodes directly adjoining the respective first semiconductor mesa.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description and onviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification. The drawings illustrate the embodiments ofthe present invention and together with the description serve to explainprinciples of the embodiment. Other embodiments and intended advantageswill be readily appreciated as they become better understood byreference to the following detailed description.

FIG. 1A is a schematic cross-sectional view of a portion of asemiconductor substrate after providing an etch mask.

FIG. 1B is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 1A after providing openings in a capping layerby using the etch mask.

FIG. 1C is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 1B after forming recesses between first celltrench structures and first semiconductor mesas.

FIG. 1D is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 1C after providing solid contact structuresfilling the openings and the recesses.

FIG. 2 is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 1C after providing contact structures withvoids in the openings and the recesses.

FIG. 3A shows a portion of a semiconductor substrate after providingrecesses between a first buried electrode and first semiconductor mesas.

FIG. 3B shows the semiconductor substrate portion of FIG. 3A afterwidening the recesses.

FIG. 3C illustrates the semiconductor substrate portion of FIG. 2B afterproviding contact structures in the openings and widened recesses.

FIG. 4A is a schematic perspective view of a portion of semiconductordevice in accordance with an embodiment related to an IGBT.

FIG. 4B illustrates a cross section of the semiconductor devices of FIG.4A along section line B.

FIG. 4C illustrates a section of the semiconductor devices of FIG. 4Aalong cross section line C.

FIG. 4D illustrates a section of the semiconductor devices of FIG. 4Aalong cross section line D.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which are shownby way of illustrations specific embodiments in which the invention maybe practiced. It is to be understood that other embodiments may beutilized and structural or logical changes may be made without departingfrom the scope of the present invention. For example, featuresillustrated or described for one embodiment can be used on or inconjunction with other embodiments to yield yet a further embodiment. Itis intended that the present invention includes such modifications andvariations. The examples are described using specific language thatshould not be construed as limiting the scope of the appending claims.The drawings are not scaled and are for illustrative purposes only. Forclarity, the same elements have been designated by correspondingreferences in the different drawings if not stated otherwise.

The terms “having”, “containing”, “including”, “comprising” and the likeare open and the terms indicate the presence of stated structures,elements or features but not preclude additional elements or features.The articles “a”, “an” and “the” are intended to include the plural aswell as the singular, unless the context clearly indicates otherwise.The term “electrically connected” describes a permanent low-ohmicconnection between electrically connected elements, for example a directcontact between the concerned elements or a low-ohmic connection via ametal and/or highly doped semiconductor. The term “electrically coupled”includes that one or more intervening element(s) adapted for signaltransmission may be provided between the electrically coupled elements,for example elements that are controllable to temporarily provide alow-ohmic connection in a first state and a high-ohmic electricdecoupling in a second state.

The Figures illustrate relative doping concentrations by indicating “−”or “+” next to the doping type “n” or “p”. For example, “n⁻” means adoping concentration that is lower than the doping concentration of an“n”-doping region while an “n⁺”-doping region has a higher dopingconcentration than an “n”-doping region. Doping regions of the samerelative doping concentration do not necessarily have the same absolutedoping concentration. For example, two different “n”-doping regions mayhave the same or different absolute doping concentrations.

FIGS. 1A to 1D refer to a semiconductor substrate 500 a consisting of orcontaining a semiconductor layer 100 a of a single-crystallinesemiconductor material. The single-crystalline semiconductor materialmay be silicon Si, silicon carbide SiC, germanium Ge, a silicongermanium crystal SiGe, gallium nitride GaN or gallium arsenide GaAs.The semiconductor substrate 500 a may be a silicon wafer from which aplurality of identical semiconductor dies is obtained. The semiconductorlayer 100 a has a planar first surface 101 and a second surface 102parallel to the first surface 101. The normal to the first and secondsurfaces 101, 102 defines a vertical direction and directions orthogonalto the vertical direction are lateral directions.

In at least a portion of the semiconductor substrate 500 a, a firstlayer of a first conductivity type may be formed that directly adjoinsthe first surface 101. The first layer of the first conductivity typemay form a planar interface with a layer of a second, complementaryconductivity type that separates the first layer of the firstconductivity type from a second layer of the first conductivity type.The interfaces between the layers may be parallel to the first surface101.

The first conductivity type may be the n type and the secondconductivity type may be the p type as illustrated in the Figures.According to other embodiments the first conductivity type may be the ptype and the second conductivity type may be the n type. Outside theillustrated portion, the semiconductor layer 100 a may include furtherimpurity zones, intrinsic zones, as well as dielectric and conductivestructures that may be configured to form electric circuits.

First and second cell trench structures 510, 520 extend from the firstsurface 101 into the semiconductor substrate 500 a, wherein buried edgesof the first and second cell trench structures have a greater distanceto the first surface 101 than a pn junction between the layer of thesecond conductivity type and the second layer of the first conductivitytype. The cell trench structures 510, 520 provide segments in the firstlayer of the first conductivity type and the layer of the secondconductivity type such that semiconductor mesas 150 between the firstand second cell trench structures 510, 520 have a layered structure withsource zones 110 of the first conductivity type directly adjoining thefirst surface 101 and body zones 115 of the second conductivity typeseparating the source zones 110 from portions of a drift layer 120 ofthe first conductivity type.

The first cell trench structures 510 include at least a first buriedelectrode 515 and a first insulator layer 516 separating the firstburied electrode 515 from the semiconductor material of thesemiconductor substrate 500 a outside the first and second cell trenchstructures 510, 520.

Each second cell trench structure 520 includes a second buried electrode525 and a second insulator layer 526 separating the second buriedelectrode 525 from the semiconductor material of the semiconductorsubstrate 500 a outside the cell trench structures 510, 520. At leastone of the first and second cell trench structures 510, 520 may includea further buried electrode dielectrically insulated from the respectivefirst or second buried electrode 515, 525.

The first and second cell trench structures 510, 520 may have the samevertical and lateral dimensions. According to other embodiments thefirst cell trench structures 510 are wider or narrower than the secondcell trench structures 520. Alternatively or in addition, the verticalextension of the first cell trench structures 510 exceeds or falls belowthe vertical extension of the second cell trench structures 520.According to an embodiment, the vertical extension of both the first andthe second cell trench structures 510, 520 may be in a range from 500 nmto 20 μm, e.g. in a range from 2 μm to 7 μm.

The first and second buried electrodes 515, 525 and, if applicable, thefurther buried electrode(s) may be provided from one or more conductivematerials including polycrystalline silicon (polysilicon), which may beheavily doped, metal silicides, carbon C, metals, e.g. copper ortungsten, metal alloys, metal nitrides, metal silicides or other metalcompounds, e.g. titanium nitride TiN, titanium tungstenide TiW, tantalumnitride TaN and others. For example, the first, the second, or bothburied electrodes 515, 516 have a layered structure including two ormore layers of the above-mentioned materials. The first and secondburied electrodes 515, 516 may have the same structure and may containthe same materials or may have different structures and/or containdifferent materials.

The first and second insulator layers 516, 526 may have the samethickness or may have different thicknesses. For example, the firstinsulator layer 516 may be thicker than the second insulator layer 526.The first and second insulator layers 516, 526 may be based on the samematerials or may consist of or may include different materials such assemiconductor oxides, e.g. silicon oxide, silicon nitride, alumina, andhafnium oxide, by way of example. According to an embodiment, at leastone of the first and second insulator layers 516, 526 has a layeredstructure including one or more different dielectric materials. Athickness of the first and second insulator layers may be between 30 nmand 200 nm, e.g. in the range between 80 nm and 120 nm. The second celltrench structure 520 may include a capping dielectric 210 between thefirst surface 101 and the second buried electrode 525. The first celltrench structure 510 may or may not include a structure corresponding tothe capping dielectric 210.

The first and second buried electrodes 515, 525 may be electricallyconnected to each other. According to the illustrated embodiment thefirst and second buried electrodes 515, 525 are electrically separatedfrom each other and can be connected to different signals or potentials.A potential applied to the second buried electrodes 525 may control thecharge carrier distribution in the adjoining body zones 115 such thatalong the second insulator layers 526 a conductive channel may be formedwhen the potential applied to the second buried electrodes 525 exceeds apredefined threshold voltage. Sections of the second insulator layers526 adjoining the body zones 115 are effective as gate dielectrics.

A capping layer 220 is provided on the first surface 101 and covers thefirst and second cell trench structures 510, 520 as well as thesemiconductor mesas 150. The capping layer 220 includes one or moredielectric layers, each layer provided, for example, from depositedsemiconductor oxide, for example a silicon oxide generated by using TEOS(tetraethyl orthosilicate) as precursor material, other silicon oxides,silicon nitride, or silicon oxynitride. The thickness of the cappinglayer 220 may be approximately uniform and may range from about 100 nmto 1 μm, by way of example.

A photo resist layer is deposited on the capping layer 220 and patternedby photolithographic techniques to form an etch mask 410.

FIG. 1A shows the capping layer 220 covering the first and second celltrench structures 510, 520 and the semiconductor mesas 150 between thefirst and second cell trench structures 510, 520. Mask openings 405 inthe etch mask 410 expose portions of the capping layer 220 in thevertical projection of first vertical sections of the first insulatorlayers 516, wherein the first vertical sections adjoin suchsemiconductor mesas 150 that separate first and second cell trenchstructures 510, 520. The mask openings 405 also expose portions of thecapping layer 220 in the vertical projection of portions of thesemiconductor mesas 150 directly adjoining the concerned sections of thefirst insulator layers 516 as well as portions of the capping layer 220in the vertical projection of a portion of the first buried electrode515 directly adjoining the concerned sections of the first insulatorlayer 516.

The etch mask 410 covers portions of the capping layer 220 in thevertical projection of the second cell trench structures 520 as well asportions of the capping layer 220 in the vertical projection of secondvertical sections of the first insulator layers 516 adjoiningsemiconductor mesas 150 between first cell trench structures 510. Usingthe etch mask 410, a predominantly anisotropic etch recesses the exposedportions of the capping layer 220. The etch removes the material of thecapping layer 220 at a higher rate than the single crystallinesemiconductor material of the semiconductor mesas 150 and the materialof the first buried electrodes 515. The etch process may include anendpoint detection sensitive to reaching at least one of thesemiconductor mesas 150, the first vertical sections of the firstinsulator layer 516, and the first buried electrode 515.

FIG. 1B shows openings 305 x in the capping layer 220 after the etchinghas reached the first surface 101. The openings 305 x expose the firstvertical sections of the first insulator layers 516, portions of thefirst buried electrodes 515 directly adjoining the concerned sections ofthe first insulator layers 516 and portions of semiconductor mesas 150directly adjoining the concerned sections of the first insulator layers516.

After detecting the endpoint, an in-situ over etch may be performed fora predefined time to recess the exposed first vertical sections of thefirst insulator layers 516. The recess etch removes exposed portions ofthe first insulator layers 516 up to a second distance to the firstsurface 101, which is greater than a first distance between the firstsurface 101 and the interface between the source and body zones 110, 115and which is smaller than a third distance between the first surface 101and the interface between the body zones 115 and the drift layer 120.The second distance may be at least 200 nm and at most 1 μm, e.g.between 400 μm and 600 μm. The material of the first insulator layer 516is recessed at a removal rate that may be at least five times theremoval rate for the semiconductor material and/or the material of thefirst buried electrode 515.

FIG. 1C shows the resulting recesses 305 y between the concerned firstburied electrodes 515 and the concerned semiconductor mesas 150. Due tothe selectivity of the etch of the insulator layer 516, the recesses areself-aligned within the openings 305 x to the first cell trenchstructures 510 and the concerned semiconductor mesas 150. In the layout,the width of the semiconductor mesas 150 may be further reduced, e.g. tobelow 400 nm.

Whereas conventional approaches provide a first etch process through thematerial of a capping layer and a second etch process for providingcontact grooves in the semiconductor materials of semiconductor mesasand buried electrodes, the present embodiment provides one combined etchfor openings in the capping layer and recesses along the semiconductormesas 150. The combined etch may be performed as one in-situ process atthe same tool.

One or more conductive materials are deposited to form a first electrodestructure 310 on the side of the semiconductor substrate 500 a definedby the first surface 101 as well as contact structures 305 electricallyconnecting the first electrode structure 310 with the first buriedelectrodes 515, the body zones 115 and the source zones 110 of thesemiconductor mesas 150 that separate first and second cell trenchstructures 510, 520. Providing the first electrode structure 310 mayinclude successive deposition of one or more conductive materials.

According to an embodiment, a barrier layer 311 having a uniformthickness in the range of 5 nm to 100 nm may be deposited. The batherlayer may bar metal atoms from diffusing into the semiconductorsubstrate 500 a and may be a layer of titanium nitride TiN, tantalumnitride TaN, titanium tungstenide TiW, titanium Ti or tantalum Ta, ormay include these materials.

A main layer 312 may be deposited on the barrier layer 311. The mainlayer 312 may consist of or contain tungsten or tungsten based metalssuch as titanium tungstenide TiW, heavily doped polysilicon, carbon C,aluminum Al, copper Cu or alloys of aluminum and copper, such as AlCu orAlSiCu. At least one of the layers may be provided with a porousstructure or may be deposited in a way to form voids or small cavitieswithin the recesses 305 y and/or the openings 305 x. Voids and cavitiesin the recesses 305 y, and openings 305 x reduce mechanical stress.

FIG. 1D shows the first electrode structure 310 including the barrierlayer 311 and the main layer 312. The thickness of the barrier layer 311may be less than a half of the width of the recess 305 y in FIG. 1C.According to another embodiment, the barrier layer 311 fills the recess305 y completely. The materials of the main layer 312 and the barrierlayer 311 may fill the openings 305 x in the capping layer 220 and therecesses 305 y in the semiconductor portion 100 completely to form solidcontact structures 315 as shown in FIG. 1D.

FIG. 2 is related to other embodiments with the main layer 312 leavingvoids 395 in the recesses 305 y and the openings 305 x.

FIGS. 3A to 3C refer to an embodiment that includes a widening of therecesses 305 y. In first sections of first insulator layers 516 betweenfirst buried electrodes 515 and such semiconductor mesas 150 that areformed between first and second cell trench structures 510, 520 and thatare exposed by openings 305 x in a capping layer 220, recesses 305 y maybe formed between the first buried electrodes 515 and the concernedsemiconductor mesas 150. An etch selectivity at which the material ofthe buried first electrode 515 is removed with respect to the materialof the semiconductor mesa 150 may be at least 5:1. The etch mask may beremoved.

FIG. 3A shows the openings 305 x in the capping layer 220 and therecesses 305 y between the first buried electrodes 515 and the concernedsemiconductor mesas 150. Contact openings 305 include an opening 305 xand a recess 305 y, respectively. A first etch step forms the opening305 in the capping layer 220 and may stop at the first surface 101. Asecond etch step that may use the same etch process over-etches thefirst insulator layer 316 for a predetermined time. Using a differentetch process, a third etch step widens at least the openings of therecesses 305 y at the expense of either the adjoining semiconductormesas 150 or the adjoining portions of the first buried electrodes 515or both. For example, a short isotropic silicon etch may removepolycrystalline material, which may be used for the first buriedelectrodes 515, at a higher etch rate than the single crystallinesemiconductor material of the semiconductor mesas 150.

According to another embodiment, a first etch process forms the openings305 in the capping layer 220 and stops at the first surface 101. Asecond etch step forms wide recesses 305 y by using an etch process withlower selectivity than the first etch process such that a certain amountof the first buried electrodes 515 is recessed contemporaneously withthe material of the first insulator layer 516. As a result, the width ofthe semiconductor mesas 150 can be essentially maintained such that achannel portion along the second cell trench structure 520 remainsunaffected from processes applied at the recesses 305 y.

According to another embodiment, the etch selectivity of the process forgenerating the recesses 305 y is gradually reduced with time such thatthe sidewall angles of the recess 305 y become less steep. In bothcases, the etch rate may be higher in the polycrystalline siliconmaterial, which may be used for the first buried electrodes 515, than inthe single crystalline semiconductor material of the semiconductor mesas150. Processes widening the recesses 305 y ease the later filling of therecesses 305 y with the one or more contact materials withoutsignificantly reducing the dimensions of the semiconductor mesas 150.

According to an embodiment, an implant may be performed through thesidewalls of the recesses 305 y to reduce a contact resistance to thebody zones 115 and the risk of latch-up effects. For example, a BF2implant may be performed. The implant may be activated through an RTA(rapid thermal anneal) to form heavily doped contact zones 117 along thesidewall portions of the semiconductor mesas 150 exposed by the widenedrecesses 305 y. The contact zones 117 have the second conductivity typeand do not reach the second cell trench structures 520 such that avariation of a threshold voltage due to impurities of the BF2 implantreaching the channel along the second insulator layer 526 can beavoided.

According to another embodiment, a plasma implant may be performedthrough the sidewalls of the widened recesses 305 y to form conformalcontact zones 117. Since the plasma implant counter-dopes portions ofthe source zones 110, the source zones 110 are provided with asufficient high net impurity concentration.

FIG. 3B shows an angled implant 380 for introducing impurities of thesecond conductivity type into exposed sidewall portions of thesemiconductor mesas 150 and the heavily doped contact zones 117 of thesecond conductivity type emerging from the angled implant 380 afteranneal. In the case of tapered recesses 305 y, the implant 380 may be anorthogonal implant perpendicular to the first surface 101. Otherwise,the implant angle with respect to the normal may be greater 0 degrees.

A barrier layer 311 may be deposited on the capping layer 220, whereinthe barrier layer 311 lines the combined contact openings 305. A mainlayer 312 is deposited that may fill the contact openings 305 completelyor that may leave voids in the contact openings 305.

FIG. 3C shows the first electrode structure 310 and the contactstructures 315 formed in the contact openings 305. A slope of thecontact openings 305 at a side oriented to the concerned semiconductormesa 150 is steeper than a slope at the opposing side oriented to theconcerned first buried electrode 515.

FIGS. 4A to 4D illustrate a semiconductor device 500 obtained from oneof a plurality of identical semiconductor dies processed as a portion ofthe semiconductor substrate 500 a of FIGS. 1A to 1D. The semiconductordevice 500 may be a power switching device, e.g. an IGBT (insulated gatebipolar transistor) e.g. a PT-IGBT (punch through IGBT) or an IGFET.

The semiconductor device 500 includes a semiconductor portion 100 with afirst surface 101 and a second surface 102 parallel to the first surface101. The semiconductor portion 100 is provided from a single-crystallinesemiconductor material, for example silicon Si, silicon carbide SiC,germanium Ge, a silicon germanium crystal SiGe, gallium nitride GaN orgallium arsenide GaAs. A minimum distance between the first and secondsurfaces 101, 102 is selected to achieve a specified voltage blockingcapability of the drift zone 120, for example 90 to 110 μm for a 1200 Vblocking IGBT. Other embodiments related to higher blocking devices orPT-IGBT device approaches may provide semiconductor portions 100 with athickness of several 100 μm distances between 101 and 102. Low voltageIGFETs may be thinner, e.g. at least some 10 μm.

The semiconductor portion 100 may have a rectangular shape with an edgelength in the range of several millimeters. The normal to the first andsecond surfaces 101, 102 defines a vertical direction and directionsorthogonal to the normal direction are lateral directions.

First and second cell trench structures 510, 520 extend from the firstsurface 101 into the semiconductor portion 100. The first and secondcell trench structures 510, 520 may have the same vertical dimensionsand the same lateral dimensions. According to other embodiments, thelateral and/or vertical dimensions of the first and second cell trenchstructures 510, 520 differ from each other. The vertical extension maybe in the range from 500 nm to 20 μm, e.g. from 2 μm to 7 μm. Thelateral width may be less than 2 μm, e.g. less than 1.2 μm.

The first cell trench structures 510 comprise first buried electrodes515 and first insulator layers 516 separating the first buriedelectrodes 515 from the semiconductor material outside the first andsecond cell trench structures 510, 520. The first insulator layers 516may have a uniform thickness in a range from 50 nm to 150 nm, e.g.between 80 nm and 120 nm, by way of example. The first cell trenchstructures 510 may or may not include further conductive structures,e.g. a further electrode dielectrically insulated from the first buriedelectrodes 515.

The second cell trench structures 520 include second buried electrodes525 and second insulator layers 526 dielectrically insulating the secondburied electrodes 525 from the semiconductor material outside the firstand second cell trench structures 510, 520. The second cell trenchstructures 520 may include a further conductive structure, for example afurther electrode dielectrically insulated from the second buriedelectrodes 525. The number of first and second cell trench structures510, 520 may be equal. Other embodiments provide more first cell trenchstructures 510 than second cell trench structures 520. For example, atleast two first cell trench structures 510 are provided between twosecond cell trench structures 520, respectively. The semiconductor mesas150 between second cell trench structures 520 may or may not beconnected to the source potential.

The first and second cell trench structures 510, 520 may be parallelstripes arranged in a regular pattern. According to other embodiments,the lateral cross-sectional areas of the cell trench structures 510, 520may be circles, ellipses, ovals or rectangles, e.g. squares, with orwithout rounded corners, or rings. For example, two or three of thefirst and second cell trench structures 510, 520 may form an arrangementwith two or three concentric rings, wherein the rings may be circles,ellipses, ovals, or rectangles, e.g. squares with or without roundedcorners.

IGBT cells may be formed in the semiconductor portion 100 at a sideoriented to the first surface 101, wherein active areas of the IGBTcells are formed in first semiconductor mesas 150 a separating one firstcell trench structure 510 and one second cell trench structure 520,respectively. In the first semiconductor mesas 150 a, source zones 110of the first conductivity type may directly adjoin the first surface101. The source zones 110 form first pn junctions with body zones 115 ofthe second conductivity type, wherein interfaces between the source andbody zones 110, 115 run approximately parallel to the first surface 101at a first distance d1. The body zones 115 form second pn junctions witha drift layer 120 of the first conductivity type at a third distance d3to the first surface 101. The first and second cell trench structures510, 520 extend through the source zones 110 and the body zones 115 intothe drift layer 120.

The illustrated embodiment refers to a fieldstop IGBT and thesemiconductor portion 100 includes a collector layer 130 that directlyadjoins the second surface 102. The collector layer 130 may be acontiguous layer of the second conductivity type. According to otherembodiments related to, e.g. reverse conducting IGBTs, the collectorlayer 130 may include first portions of the first conductivity type andsecond portions of the second conductivity type, wherein the first andsecond portions alternate in one lateral direction or in both lateraldirections. A mean net impurity concentration in the collector layer 130may be at least 1×10¹⁶ cm⁻³, for example at least 5×10¹⁷ cm⁻³.

A second electrode structure 320 directly adjoins the second surface102. The second electrode structure 320 is electrically connected to thecollector layer 130 and may consist of or contain, as mainconstituent(s) aluminum Al, copper Cu, or alloys of aluminum or copper,such as AlSi, AlCu or AlSiCu. According to other embodiments, thecollector electrode 320 may contain one, two, three or more sub-layers,wherein each sub-layer contains, as main constituent(s), at least one ofnickel Ni, titanium Ti, silver Ag, gold Au, tungsten W, platinum Ptand/or palladium Pd. For example, a sub-layer may contain a metalsilicide, a metal nitride, or a metal alloy containing Ni, Ti, Ag, Au,W, Pt, and/or Pd. For IGBTs, the second electrode structure 320 providesa collector electrode that may provide or may be electrically connectedto a collector terminal C of the semiconductor device 500.

In the drift layer 120, a field stop layer 128 may be provided betweenthe collector layer 130 and a drift zone 121. A mean net impurityconcentration in the field stop layer 128 may be between 5×10¹⁵ cm⁻³ and1×10¹⁷ cm⁻³. The mean net impurity concentration in the drift zone 121is lower than in the field stop layer 128. According to an embodiment,the mean net impurity concentration in the field stop layer 128 exceedsat least five times the mean net impurity concentration in the driftzone 121. The mean net impurity concentration in the drift zone 121 maybe between 5×10¹² cm⁻³ and 5×10¹⁴ cm⁻³, by way of example. For IGFETs, aheavily doped contact layer of the first conductivity type replaces thecollector layer 130 and the second electrode structure 320 provides adrain electrode that may provide or may be electrically connected to adrain terminal of the semiconductor device 500.

The second buried electrodes 525 provide insulated gate electrodes Ga. Apotential applied to the insulated gate electrodes Ga controls aminority charge carrier distribution in channel portions 115 a of thebody zones 115, wherein the channel portions 115 a adjoin the secondcell trench structures 520 between the source zones 110 and the driftlayer 120. If in a forward biased mode the potential applied to theinsulated gate electrodes Ga exceeds a predefined threshold voltage,inversion channels of the first conductivity type are formed in the bodyzones 115 along the second insulator layers 526, which are effective asgate dielectrics, and an on-state current flows between the source zones110 and the drift layer 120. The insulated gate electrodes Ga may beelectrically connected to a third electrode structure 330 that mayprovide or may be electrically connected or coupled to a gate terminal Gof the semiconductor device 500.

Second semiconductor mesas 150 b between first cell trench structures510 may or may not include source zones 110. In the latter case, thebody zones 115 may extend between the first surface 101 and the driftlayer 120.

The first cell trench structures 510 provide buried source electrodes Sthat may be electrically connected to an emitter terminal E of thesemiconductor device 500. The insulated gate electrodes Ga are insulatedfrom the buried source electrodes S. At least the second cell trenchstructures 520 may include a capping dielectric 210 between the firstsurface 101 and the second buried electrodes 525 to reduce an overlapbetween the insulated gate electrodes Ga and the source zones 110. Otherembodiments may provide contacts to some or all of the secondsemiconductor mesas 150 b.

A dielectric capping layer 220 may dielectrically insulate at least thesecond cell trench structures 520 and the second semiconductor mesas 150b from a first electrode structure 310 disposed at a side defined by thefirst surface 101. First contact structures 315 electrically connect thefirst electrode structure 310 with the first semiconductor mesas 150 aand such first cell trench structures 510 that directly adjoin the firstsemiconductor mesas 150 a. Second contact structures 316 electricallyconnect the first electrode structure 310 with other first cell trenchstructures 510 not directly adjoining the first semiconductor mesas 150a.

Each of the first contact structures 315 includes a first section 315 ain an opening of the capping layer 220 and a second section 315 bbetween a first semiconductor mesa 150 a and a first cell trenchstructure 510 directly adjoining the first semiconductor mesa 150 a. Thesecond section 315 b extends from the first surface 101 into thesemiconductor portion 100. A second distance d2 between the firstsurface 101 and the buried edge of the second section 315 b is greaterthan the first distance d1 and smaller than the third distance d3.

The second sections 315 b of the first contact structures 315 may haveapproximately vertical sidewalls. According to an embodiment, thesidewalls for the second sections 315 b taper with increasing distanceto the first surface 101.

According to an embodiment, first sidewalls of the second sections 315 bof the first contact structures 315 are tilted to the first surface 101and directly adjoin the first semiconductor mesas 150 a. Secondsidewalls of the second sections 315 b of the first contact structures315 may be tilted to the first surface 101 and directly adjoin the firstburied electrodes 510. The first sidewalls oriented to the firstsemiconductor mesas 150 and the second sidewalls oriented to the firstburied electrodes 510 may have identical slope angles. The secondsidewalls may deviate to a higher degree from a normal to the firstsurface 101 than the first sidewalls of the second sections 315 b of thefirst contact structures 315.

The second sections 315 b of the first contact structures 315 arelocated in the vertical projection of first sections of the firstinsulator layers 516. The first insulator layers 516 may have a uniformwidth, wherein the width of the first insulator layers 516 may be equalto or less than a width of the second sections 315 b of the firstcontact structures 315. The first contact structures 315 are deep enoughto provide a direct contact to the body zones 115.

Heavily doped contact zones 117 may be formed in the body zones 115 ofthe first semiconductor mesas 150 a along the interfaces to the firstcontact structures 315. The second electrode structure 310 as well asthe third electrode structure 330 may include at least one barrier layer311, 331, respectively. The barrier layer may have a uniform thicknessin the range of 5 nm to 100 nm and may consist of or include a layer oftitanium nitride TiN, tantalum nitride TaN, titanium tungstenide TiW,titanium Ti or tantalum Ta, by way of example. The main layers 312, 332may consist of or contain tungsten or tungsten-based metals liketitanium tungstenide TiW, heavily doped polysilicon, carbon C, aluminumAl, copper Cu or alloys of aluminum and copper, for example AlCu orAlSiCu.

The first and second contact structures 315, 316 may be solid contactstructures, may include a porous layer or may have voids as shown inFIG. 2. The source zones 110 may be provided as narrow stripes and mayalternate with portions of the body zones 115 in a lateral directionparallel to stripe shaped first and second cell trench structures 510,520.

Uncertainties and inequalities of different lithographic layersresulting in a misalignment between contact structures and semiconductormesas limit a minimal mesa width conventionally at about 600 nm.Instead, the semiconductor device 500 of FIGS. 4A to 4D facilitatesnarrowing the width of the semiconductor mesas to less than 300 nm, forexample less than 200 nm.

Further embodiments concern layout modifications of the first and secondtrench structures 510, 520 to further reduce an effective channel widthfor increasing short-circuit ruggedness, e.g. by segmenting the secondcell trench structures 520 or by increasing locally a thickness of thesecond insulator layers 526.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A semiconductor device comprising: first celltrench structures comprising a first buried electrode; second celltrench structures comprising a second buried electrode electricallyseparated from the first buried electrode, the first and second celltrench structures extending from a first surface into a semiconductorsubstrate, first semiconductor mesas separating first and second celltrench structures and comprising a source zone of a first conductivitytype and a body zone of a second conductivity type that is complementaryto the first conductivity type at a first distance to the first surface,the source zone extending along the first surface from the first celltrench structure to the second cell trench structure, and the body zonecomprising a contact zone along a sidewall portion of the firstsemiconductor mesa, second semiconductor mesas separating first celltrench structures from each other, wherein the first cell trenchstructures comprise first insulator layers and first vertical sectionsof the first insulator layers separate the first buried electrodes fromthe first semiconductor mesas; a capping layer on the first surface, thecapping layer sandwiched between an electrode structure and the secondsemiconductor mesas and structurally separating the electrode structurefrom the second semiconductor mesas; and first contact structuresphysically disconnected from the second semiconductor mesas, each firstcontact structure comprising a first section directly connected to theelectrode structure in an opening of the capping layer and a secondsection sandwiched between one of the first semiconductor mesas and oneof the first buried electrodes and directly adjoining the source and thecontact zones in the respective first semiconductor mesa, wherein thefirst cell trench structure separates the second section from aneighboring one of the second semiconductor mesas, a vertical extensionof the second section is greater than the first distance, and only onefirst contact structure is formed per first semiconductor mesa.
 2. Thesemiconductor device of claim 1, wherein first sidewalls of the secondsections of the first contact structures directly adjoin the firstsemiconductor mesas, and second sidewalls directly adjoin the firstburied electrodes and at least the second sidewalls are tilted to thefirst surface by less than 90 degrees.
 3. The semiconductor device ofclaim 2, wherein the first and second sidewalls taper with increasingdistance to the first surface and the second sidewalls deviate to ahigher degree from a normal to the first surface than the firstsidewalls of the second sections of the first contact structures.
 4. Thesemiconductor device of claim 1, further comprising second contactstructures, each second contact structure formed in an opening of thecapping layer and directly adjoining one of the first buried electrodesbetween two of the second semiconductor mesas.
 5. The semiconductordevice of claim 1, wherein the first insulator layer has a uniformwidth.
 6. The semiconductor device of claim 1, wherein a width of thefirst insulator layer is equal to or less than a width of the secondsections of the first contact structures.
 7. The semiconductor device ofclaim 1, wherein the second sections of the first contact structures arelocated in a vertical projection of first vertical sections of the firstinsulator layers.